As materials of molded products made from plastics, there have been conventionally used polystyrene, polyvinyl chloride, polypropylene and polyethylene terephthalate resins. Some of the molded products produced from these resins are excellent in transparency, but if a wrong disposal means is taken, the quantity of the waste is increased. Further, the waste of the molded articles is hardly decomposed in the natural environment, and therefore, if disposal by burying the waste under the ground is made, the waste remains in the ground semi-permanently.
On the other hand, as polymers that are thermoplastic resins and have biodegradability, lactic acid polymers, such as polylactic acid and copolymers of lactic acid and other hydroxycarboxylic acids, have been developed. The lactic acid polymers have characteristics such that they undergo biodegradation of 100% within several months to one year in the bodies of animals and that when they are placed in the soil or seawater, they start biodegradation in several weeks in the wet environment and disappear in about one to several years, and their biodegradation products become lactic acid that is harmless to the human bodies, carbon dioxide and water.
Molded articles (e.g., molded articles having three-dimensional shapes, such as bottles, unstretched films or sheets having two-dimensional shapes, unstretched filaments or yarns having one-dimensional shapes) of such lactic acid polymers are usually non-crystalline immediately after molding, and they are transparent because crystals having sizes almost equal to or larger than the wavelength of light, which cause scattering of light, are rarely present.
The transparent molded articles, however, are usually inferior in heat resistance because they have low glass transition temperature (Tg) and are non-crystalline. For example, containers of non-crystalline polylactic acid have low heat resistance though they are excellent in transparency. Therefore, hot water and microwave oven cannot be used, and their uses have been restricted. If crystallinity of such molded articles is increased in order to enhance heat resistance by filling the molding material in a mold maintained at a temperature in the vicinity of the crystallization temperature in the molding process or by heat-treating (annealing) the non-crystalline molded article after molding, crystals (e.g., spherocrystal) having sizes almost equal to or larger than the wavelength of light, which cause scattering of light, grow rapidly, and the molded article becomes opaque.
Then, it has been studied to enhance heat resistance of sheets or molded articles by adding a crystal nucleating agent to the lactic acid polymer and thereby accelerating crystallization. However, it is difficult to impart heat resistance without inhibiting transparency of the resins themselves.
For example, in Japanese Patent Laid-Open Publication No. 146170/2002 (patent document 1), it is described that a highly practical film is obtained by imparting specific crystalline property to a plasticized polylactic acid resin containing a plasticizer and a crystal nucleating agent as essential components. In this method, however, transparency is markedly lowered because particle diameters of the crystal nucleating agent added are large or the quantity of the nucleating agent added is large, and therefore, it is difficult to obtain a sheet or a molded article having excellent transparency.
In Japanese Patent Laid-Open Publication No. 278991/1997 (patent document 2), a molded article which has been imparted with transparency and crystalline properties by molding a composition comprising at least one transparent nucleating agent selected from the group consisting of an aliphatic carboxylic acid amide, an aliphatic carboxylic acid salt, an aliphatic alcohol and an aliphatic carboxylic acid ester, and a lactic acid polymer, and carrying out heat treatment during or after the molding, and a process for producing the molded article are described. In this case, however, crystallization takes a long time, and hence, such a level that the process can be carried out industrially is not reached from the viewpoint of productivity.
In Japanese Patent Laid-Open Publication No. 204143/2004 (patent document 3), technique relating to a composition comprising biodegradable polyester and layered silicate and a molded product using the composition is disclosed. In this method, however, it is difficult to finely control the dispersed state of the layered silicate to such a degree that transparency is not inhibited, and it is difficult to attain high transparency.    Patent document 1: Japanese Patent Laid-Open Publication No. 146170/2002    Patent document 2: Japanese Patent Laid-Open Publication No. 278991/1997    Patent document 3: Japanese Patent Laid-Open Publication No. 204143/2004